In addition to their ability to convert chemical signals into electrical responses, neurotransmitter-gated ion channels, they are dynamic molecular entities, subject to regulation of their responsiveness over a wide temporal range. The focus of the experiments described in this proposal is on the mechanisms that regulate the neural acetylcholine receptor (AcChR) of PC12 cells, a clonal sympathetic cell line. The kinetic properties of agonist- mediated desensitization will be examined by sodium influx measurements of receptor activity. The possibility that phosphorylation-dephosphorylation reactions are involved in the development of long lasting desensitization that is characterized by slow onset and recovery rates, will be investigated by obtaining measurements, in parallel, of the extent of AcChR phosphorylation. A binding assay to quantitate nAcChRs of PC12 cells will be developed to examine whether deactivation, an agonist-induced process that produces an irreversible dimunution of the permeability response, involves a rapid loss of surface receptors. The relationship between deactivation and the receptor states involved in desensitization will be examined by kinetic analysis of the effects on the rates of each of these processes by different cholinergic antagonists. The role of other synaptic components both extracellular, e.g. substance P, and intracellular, e.g. Ca2+ and cAMP, in modulating further these agonist-mediated processes will also be evaluated by the same biochemical techniques. By examining the mechanisms of short- and long-term regulation of nAcChR by agonists and other synaptic components, the proposed studies will aid in evaluating the possible role of postsynaptic mechanisms in modulating synaptic efficacy. A better understanding of these processes will shed light on one of the most remarkable properties of the brain: its ability to be regulated by experience.